Hollow fiber membrane module

ABSTRACT

There is provided a hollow fiber membrane module including a cylindrical case having at least a discharge port for raw water in a side face thereof; a fixing member to which open ends of a large number of hollow fiber membranes forming a hollow fiber membrane bundle housed inside the cylindrical case are fixed; and a flow straightening cylinder which is provided between the discharge port for the raw water and an outer periphery of the hollow fiber membrane bundle, which is fixed to the fixing member, and which has a plurality of flow straightening holes. The flow straightening cylinder includes a large inner diameter portion and a small inner diameter portion and the small inner diameter portion is mounted to the fixing member with a lower face of the fixing member positioned at the small inner diameter portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2013/067655, filedJun. 27, 2013, which claims priority to Japanese Patent Application No.2012-151041, filed Jul. 5, 2012, the disclosures of each of theseapplications being incorporated herein by reference in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates to a hollow fiber membrane module used fora water treatment such as a water purification treatment, production ofindustrial water, a wastewater treatment, and a reverse osmosis membranepretreatment. Hollow fiber membranes used in the hollow fiber membranemodule is configured of microfiltration membranes or ultrafiltrationmembranes, for example.

BACKGROUND OF THE INVENTION

External-pressure type hollow fiber membrane modules which filtratewater from the outside of the hollow fiber membranes to the insidehollow portions have various merits including the simplicity of the sealstructure which separates raw water which has not underdone membranefiltration from filtrate obtained after membrane filtration, ease ofoperation management, etc. The greatest feature thereof resides in thatthe module can have an exceedingly large filtration membrane area perunit volume of the module. Because of this, the external-pressure typehollow membrane modules are being increasingly applied in recent yearsto water treatment processes for producing industrial water or tap waterfrom river water, lake water, groundwater, seawater, householdwastewater, or industrial wastewater.

When raw water is membrane-filtrated using the hollow fiber membranemodule, substances which are contained in the raw water and are to beremoved, such as suspended substances and organic matters, accumulate onouter surfaces of the membranes to cause a membrane clogging phenomenon.As a result, the filtration resistance of the membranes increases,shortly rendering the filtration impossible. Consequently, a generalmethod for maintaining the membrane filtration performance is toperiodically stop the membrane filtration and conduct physical cleaningof the filtration membranes.

Usually, the filtration step and the physical cleaning step areautomatically conducted repeatedly. Examples of the physical cleaninginclude air scrubbing in which air is introduced into the lower part ofthe membrane module to oscillate the membranes in water to thereby shakeoff the suspended substances adherent to the outer surfaces of themembranes, back-pressure washing (back-washing) in which water (washingwater) such as filtrate is forced into the membranes by pressure in thedirection reverse to the filtration direction of the membrane module,i.e., from the hollow portion-side to the outer surfaces of themembranes, to remove the suspended substances adherent to the membranes,and air/back-pressure simultaneous washing in which the air scrubbingand the back-pressure washing are conducted simultaneously, etc.

The hollow fiber membrane module is generally configured of acylindrical case and a hollow fiber membrane bundle housed in thecylindrical case. One end of the hollow fiber membrane bundle is fixedto one end of the cylindrical case by casting one potting material andthe other end of the hollow fiber membrane bundle is fixed to the otherend of the cylindrical case by casting the other potting material. Thehollow fiber membrane bundle used here is usually configured of a bundleof hundreds to tens of thousands of hollow fiber membranes.

In the case of external-pressure type hollow fiber membrane modules, theone ends of the respective hollow fiber membranes fixed by the onepotting material have openings configured of the hollow portions openingat an outer surface of the one potting material. The filtrate obtainedby passage and filtration of the raw water through the hollow fibermembranes flows through the hollow portions to reach the openings andthe filtrate which has passed through the openings flows to a filtrateoutlet provided to the cylindrical case. The other ends of therespective hollow fiber membranes fixed by the other potting materialusually have closed portions formed by sealing of the hollow portionsinside the other potting material.

Well known as methods for the fixing by casting a potting material are astationary method and a centrifugal method. The stationary method is amethod in which a liquid potting material, e.g., resin havingflowability, is fed with a constant delivery pump or the like from belowthe hollow fiber membrane bundle and the potting material is solidified(cured) at or near tip ends of the respective hollow fiber membranes.The centrifugal method is a method in which a liquid potting material ismoved to the end of the case by centrifugal force and solidified(cured). The solidified potting material serves as a fixing member forfixing the respective hollow fiber membranes to the inside of thepotting material and fixing the hollow fiber membrane bundle to thecylindrical case.

In either method, however, the resin having the flowability creeps upalong the outer surfaces of the hollow fiber membranes to a height ofabout several millimeters to several centimeters at the interfacebetween the hollow fiber membranes and the potting material to form anuneven resin surface and is solidified in this state. In the case wherethe hollow fiber membranes which have been fixed to the resin (fixingmembers) solidified in such a state receive oscillations generatedduring air scrubbing, local stress is imposed on the hollow fibermembranes at the uneven resin surfaces, resulting in hollow fibermembrane rupture in some cases.

To an upper side face of the cylindrical case housing the hollow fibermembrane bundle, a nozzle used for discharge of circulating water andwastewater is provided. The resin surface (lower face of the upperfixing member), on an inner side with respect to an axial direction ofthe cylindrical case, of the upper potting material (upper fixingmember) is formed at substantially the same height position as an upperend of an inner flow path of the upper discharge nozzle so that thecirculating water and the wastewater can be discharged efficiently.

If the resin surface (lower face of the upper fixing member), on theinner side of the cylindrical case, of the upper fixing member fixingthe respective hollow fiber membranes is disposed above the upper end ofthe inner flow path of the upper discharge nozzle, air or suspendedsubstances may stay in the space between the lower face of the upperfixing member and the upper end of the inner flow path of the upperdischarge nozzle, resulting in degradation of turbidity removingperformance and reduction in an effective membrane area of the hollowfiber membranes for filtration in this case.

Around the upper portion of the hollow fiber membrane bundle, a flowstraightening cylinder having flow straightening holes is disposed and adischarge flow path communicating with the upper discharge nozzle isformed.

In order to further enhance the air scrubbing effect, Patent Document 1proposes provision of slack to hollow fiber membranes housed in acylindrical case. By fixing the opposite ends of the hollow fibermembranes to the bonded/fixed portions at opposite ends of the hollowfiber membranes while providing the slack to the hollow fiber membranesbetween bonded/fixed portions, the hollow fiber membranes oscillate toproper degrees during the air scrubbing and effective scrubbing can beconducted. However, during water discharge at a high flow rate, thehollow fiber membranes pushed by the discharged water and the air flowclose the flow straightening holes to thereby cause a pressure loss orthe hollow fiber membranes are pushed against the flow straighteningholes and damaged.

To solve the problems, Patent Document 2 proposes a hollow fibermembrane module in which grooves connecting flow straightening holes areformed on an inner face of a flow straightening cylinder providedbetween a water discharge nozzle and a hollow fiber membrane bundle.

PATENT DOCUMENTS

Patent Document 1: Japanese Patent Laid-open Publication No. 10-000339A

Patent Document 2: WO2008/035593A1

SUMMARY OF THE INVENTION

Problems of conventional techniques will be described below withreference to FIG. 4 by taking a hollow fiber membrane module using aconventional flow straightening cylinder as an example. Hollow fibermembrane modules having the same structure as that of the conventionalhollow fiber membrane module shown in FIG. 4 are described in manydocuments. Examples of such hollow fiber membrane modules include thehollow fiber membrane module shown in FIG. 1 in Patent Document 2.

In FIG. 4, a cylindrical case 1 in the conventional hollow fibermembrane module M4 is configured of a cylindrical case main body 3, anupper socket 4 a mounted to an outer peripheral face of an upper end ofthe case main body 3, and a lower socket 4 b mounted to an outerperipheral face of a lower end of the case main body 3. A discharge port9 for raw water is provided to the upper socket 4 a and a feed port 10for the raw water is provided to the lower socket 4 b. A hollow fibermembrane bundle 2 cut at opposite ends to have the same lengths isinserted into the cylindrical case 1. The hollow fiber membrane bundle 2is configured of a bundle of a large number of hollow fiber membranes 2a.

Upper ends of the large number of hollow fiber membranes 2 a arebonded/fixed by resin and the resin forms an upper fixing member 13 a ofthe hollow fiber membrane bundle 2. A side peripheral face of the upperfixing member 13 a is fixed to an inner peripheral face of an upper endof the upper socket 4 a. The upper ends of the large number of hollowfiber membranes 2 a fixed to the upper fixing member 13 a are open tothe outside at an upper surface 13 aUS of the upper fixing member 13 aand each of the hollow fiber membranes 2 a has an opening 2 aO. A lowersurface 13 aLS of the upper fixing member 13 a is positioned inside thecylindrical case 1.

Lower ends of the large number of hollow fiber membranes 2 a arebonded/fixed by resin and the resin forms a lower fixing member 13 b ofthe hollow fiber membrane bundle 2. A side peripheral face of the lowerfixing member 13 b is fixed to an inner peripheral face of a lower endof the lower socket 4 b. Hollow portions in the lower ends of the largenumber of hollow fiber membranes 2 a fixed to the lower fixing member 13b are closed inside the lower fixing member 13 b with resin used to formthe lower fixing member 13 b and each of the hollow fiber membranes 2 ahas a closed portion 2 aC. An upper surface 13 bUS of the lower fixingmember 13 b is positioned inside the cylindrical case 1 and a lowersurface 13 bLS of the lower fixing member 13 b is positioned outside thecylindrical case 1.

To a peripheral face of the hollow fiber membrane bundle 2 at a positionfacing the feed port 10, a lower flow straightening cylinder 6 b havinga large number of flow straightening holes 8 b is provided. Between aninner peripheral face of the lower socket 4 b and an outer peripheralface of the lower flow straightening cylinder 6 b, a lower annular flowpath 7 b is formed.

The raw water is fed from the feed port 10 provided to the lower socket4 b into the cylindrical case 1, passes from the lower annular flow path7 b through the flow straightening holes 8 b provided to the lower flowstraightening cylinder 6 b, and flows along outer surfaces of therespective hollow fiber membranes 2 a.

The raw water may be fed into the cylindrical case 1 from a second feedport 12 formed in a lower cap 5 b provided to a lower end of thecylindrical case 1. In this case, the raw water fed from the second feedport 12 passes through through holes 14 provided to the lower fixingmember 13 b to penetrate the lower fixing member 13 b in a verticaldirection and flows along outer surfaces of the respective hollow fibermembranes 2 a.

The raw water flowing along the outer surfaces of the respective hollowfiber membranes 2 a permeates through membrane walls of the respectivehollow fiber membranes 2 a to reach the hollow portions of the hollowfiber membranes to be filtrate. The filtrate passes through the openings2 aO of the respective hollow fiber membranes 2 a and is collected intoan upper cap 5 a provided to an upper end of the cylindrical case 1 andthen taken out of the hollow fiber membrane module M4 from a filtrateoutlet 11 provided to the upper cap 5 a.

In a dead-end filtration method, the discharge port 9 for the raw wateris closed and only the filtrate is taken out of the filtrate outlet 11.In a cross-flow method, on the other hand, the remaining raw water(concentrated water) which has not permeated through the hollow fibermembranes passes through flow straightening holes 8 a in an upper flowstraightening cylinder 6 a provided to a peripheral face of the hollowfiber membrane bundle 2 at a position facing the discharge port 9 forthe raw water and having the large number of flow straightening holes 8a, passes through an upper annular flow path 7 a provided between aninner peripheral face of the upper socket 4 a and an outer peripheralface of the upper flow straightening cylinder 6 b, and is discharged outof the hollow fiber membrane module M4 from the discharge port 9 for theraw water.

If the conventional upper flow straightening cylinder 6 a is used, whenthe raw water flowing along the outer surfaces of the respective hollowfiber membranes 2 a passes through the flow straightening holes 8 a frominside the upper flow straightening cylinder 6 a and is discharged fromthe discharge port 9 through the upper annular flow path 7 a, the hollowfiber membranes 2 a positioned near an inner peripheral face of theupper flow straightening cylinder 6 a is pushed by a flow of thedischarged water against the flow straightening holes 8 a and areasaround the flow straightening holes 8 a and the flow straightening holes8 a are liable to be closed in the case in which an amount of dischargedwater per unit time is excessively large.

After the end of a filtration step for a certain time, the air scrubbingin which air is fed from the second feed port 12 to shake off thesuspended substances accumulating in the hollow fiber membrane module M4and the back-pressure washing in which the filtrate is forced to flowfrom the side of the filtrate outlet 11 to the raw water side areconducted successively or both of them are conducted simultaneously. Inthese washing steps, when the discharged water or the air is caused toflow out from the discharge port 9, the hollow fiber membranes 2 a arepushed against the flow straightening holes 8 a and the areas around theflow straightening holes 8 a and the flow straightening holes 8 a areliable to be closed in the same way as in the above description.

In the hollow fiber membrane module shown in Patent Document 2, by usingthe structure described in this document, it is possible to prevent thehollow fiber membranes near the discharge port 9 from closing the flowstraightening holes 8 a provided to the upper flow straighteningcylinder 6 a. However, especially when the water discharge at a highflow rate is conducted, the hollow fiber membranes positioned at anouter peripheral portion of the hollow fiber membrane bundle 2 and thegrooves provided to the inner surface of the upper flow straighteningcylinder 6 a come in contact with each other, which shaves and damagesthe hollow fiber membranes.

It is an object of the present invention to provide a hollow fibermembrane module with which a pressure loss in water discharge at a highflow rate can be reduced while damage to hollow fiber membranes can beprevented.

In order to solve this object, a hollow fiber membrane module accordingto an aspect of the present invention is formed as follows.

According to the aspect of the present invention, there is provided ahollow fiber membrane module including: a cylindrical case having onenozzle at the minimum in a side face thereof; a hollow fiber membranebundle provided in the cylindrical case; and a flow straighteningcylinder provided between the one nozzle at the minimum and an outerperiphery of the hollow fiber membrane bundle and having a plurality offlow straightening holes, in which the hollow fiber membrane bundleincludes a plurality of hollow fiber membranes, that are open at oneends and sealed at the other ends, or at least one U-shaped hollow fibermembrane (FIG. 5), that is open at opposite ends, and includes a hollowfiber membrane open end fixing member for fixing the open end of each ofthe hollow fiber membranes, a side peripheral face of the hollow fibermembrane open end fixing member is joined to an inner peripheral face ofthe cylindrical case, each of the hollow fiber membranes passes throughthe hollow fiber membrane open end fixing member, each of the openingsis positioned at an outer surface of the hollow fiber membrane open endfixing member, the outer surface positioned outside the cylindricalcase, and the flow straightening cylinder is fixed to the hollow fibermembrane open end fixing member, wherein the flow straightening cylinderincludes a large inner diameter portion and a small inner diameterportion having a smaller inner diameter than an inner diameter of thelarge inner diameter portion at least at a part in an axial direction ofthe flow straightening cylinder and an inner surface of the hollow fibermembrane open end fixing member positioned on an opposite side from theouter surface and inside the cylindrical case is positioned at the smallinner diameter portion of the flow straightening cylinder.

According to the aspect of the invention, preferably, the hollow fibermembrane bundle includes a plurality of hollow fiber membranes that areopen at one ends and sealed at the other ends, the sealed end of each ofthe hollow fiber membranes is fixed to a hollow fiber membrane sealedend fixing member, a side peripheral face of the hollow fiber membranesealed end fixing member is joined to an inner peripheral face of thecylindrical case, and the hollow fiber membrane sealed end fixing memberhas an inner surface positioned inside the cylindrical case and an outersurface positioned outside the cylindrical case.

According to the aspect of the invention, preferably, a length, in anaxial direction of the cylindrical case, of the small inner diameterportion from the inner surface of the hollow fiber membrane open endfixing member toward the large inner diameter portion is in the range of3 mm to 40 mm.

According to the aspect of the invention, preferably, when across-sectional area of the small inner diameter portion of the flowstraightening cylinder is SA and a cross-sectional area of the largeinner diameter portion is LA, a ratio between SA and LA satisfies arelationship, 0.6≦SA/LA≦50.9.

According to the aspect of the invention, preferably, all of theplurality of flow straightening holes of the flow straightening cylinderare provided at the large inner diameter portion.

According to the aspect of the invention, preferably, the flowstraightening cylinder has an inner diameter length transition zone inwhich an inner diameter of the flow straightening cylinder changes froman inner diameter satisfying the cross sectional area SA of the smallinner diameter portion to an inner diameter satisfying thecross-sectional area LA of the large inner diameter portion between thesmall inner diameter portion and the large inner diameter portion.

According to the aspect of the invention, preferably, when the innerdiameter of the large inner diameter portion is LD, the inner diameterof the small inner diameter portion is SD, a hollow fiber membranelength between the inner surface of the hollow fiber membrane open endfixing member and the inner surface of the hollow fiber membrane sealedend fixing member is FL, and a distance, in the axial direction of thecylindrical case, between the inner surface of the hollow fiber membraneopen end fixing member and the inner surface of the hollow fibermembrane sealed end fixing member is CL, the flow straightening cylindersatisfies a relationship, (LD−SD)>(FL−CL)/2.

According to the aspect of the invention, preferably, a filling rate ofthe hollow fiber membranes in the hollow fiber membrane bundlepositioned at the small inner diameter portion is in the range of 30% to80%.

According to the aspect of the invention, preferably, an outer diameterof the flow straightening cylinder at a portion where the small innerdiameter portion is formed is smaller than an outer diameter of the flowstraightening cylinder at a portion where the large inner diameterportion is formed.

With the hollow fiber membrane module according to the aspect of thepresent invention, it is possible to reduce a pressure loss in waterdischarge at a high flow rate while preventing damage to the hollowfiber membranes caused by contact of the hollow fiber membranes with theflow straightening holes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic vertical cross-sectional view showing oneexample of hollow fiber membrane modules according to the invention.

FIG. 2 is a vertical cross-sectional enlarged view of a hollow fibermembrane open end fixing member (upper fixing member) and an area nearthe member of a hollow fiber membrane module according to the inventionshown in FIG. 1.

FIG. 3 is a diagrammatic vertical cross-sectional view of one example ofupper flow straightening cylinders used for the hollow fiber membranemodule according to the invention, to which lateral cross-sectionalviews of left and right inner diameter portions are added.

FIG. 4 is a diagrammatic vertical cross-sectional view of one example ofconventional hollow fiber membrane modules.

FIG. 5 is a diagrammatic vertical cross-sectional view showing oneexample of hollow fiber membrane modules according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

One embodiment of hollow fiber membrane modules according to the presentinvention is explained below while referring to FIGS. 1, 2, and 5. Thehollow fiber membrane module described here is used as a filtrationdevice for raw water for producing tap water (drinking water).

Out of respective components forming the hollow fiber membrane moduleaccording to the invention shown in FIGS. 1, 2, and 5, the samecomponents as those forming the hollow fiber membrane module shown inFIG. 4 are provided with the same reference signs as those provided tothe components used in FIG. 4 in FIGS. 1, 2, and 5.

The hollow fiber membrane module M1 shown in FIG. 1 is configured of acylindrical case 1 and a hollow fiber membrane bundle 2 provided in thecylindrical case 1. The cylindrical case 1 is configured of acylindrical case main body 3, an upper socket 4 a mounted to an outerperipheral face of an upper end of the case main body 3, and a lowersocket 4 b mounted to an outer peripheral face of a lower end of thecase main body 3.

The upper socket 4 a has a discharge port (nozzle) 9 for raw water at aportion of its peripheral face. Between the upper socket 4 a and anouter peripheral face of the hollow fiber membrane bundle 2, an upperflow straightening cylinder 6A having a plurality of flow straighteningholes 8 a is provided. Between an outer peripheral face of the upperflow straightening cylinder 6A and an inner peripheral face of the uppersocket 4 a, an upper annular flow path 7 a is formed.

The hollow fiber membrane bundle 2 is configured of a bundle of a largenumber of hollow fiber membranes 2 a. A hollow portion of each of thehollow fiber membranes 2 a is open at one end and sealed at the otherend. The open end of each of the hollow fiber membranes 2 a is fixed toa hollow fiber membrane open end fixing member (upper fixing member) 13a. The hollow fiber membrane open end fixing member (upper fixingmember) 13 a is fixed to the cylindrical case 1 by joining of a sideperipheral face of the hollow fiber membrane open end fixing member(upper fixing member) 13 a to the inner peripheral face of the uppersocket 4 a.

Each of the hollow fiber membranes 2 a penetrates the hollow fibermembrane open end fixing member (upper fixing member) 13 a and thehollow portion of each of the hollow fiber membranes 2 a is open at anupper surface (outer surface) 13 aUS of the hollow fiber membrane openend fixing member (upper fixing member) 13 a to form an opening 2 aO. Anupper end of the upper flow straightening cylinder 6 a is fixed to thehollow fiber membrane open end fixing member (upper fixing member) 13 a.

The upper flow straightening cylinder 6A has a large inner diameterportion LDP at its lower portion and a small inner diameter portion SDPhaving a smaller inner diameter SD than an inner diameter LD of thelarge inner diameter portion LDP at its upper portion in an axialdirection. A lower surface (inner surface) 13 aLS of the hollow fibermembrane open end fixing member (upper fixing member) 13 a which is onan opposite side from the upper surface (outer surface) 13 aUS of thehollow fiber membrane open end fixing member (upper fixing member) 13 aand positioned inside the cylindrical case 1 is positioned at the smallinner diameter portion SDP of the upper flow straightening cylinder 6A.

Although the hollow fiber membrane bundle 2 in which the hollow portionof each of the hollow fiber membranes 2 a is open at one end and sealedat the other end is described above, it is also possible to use a hollowfiber membrane bundle 2 in which each of hollow fiber membranes 2 a hasa U or loop shape, has opposite ends fixed to a hollow fiber membraneopen end fixing member (upper fixing member) 13 a, and the opposite endsare open on an upper surface (outer surface) 13 aUS of the hollow fibermembrane open end fixing member (upper fixing member) 13 a. One exampleof the hollow fiber membrane bundles configured of the loop-shapedhollow fiber membranes is shown in FIG. 4 in Patent Document 1.

By integrating the upper flow straightening cylinder 6A having the largeinner diameter portion LDP and the small inner diameter portion SDPdescribed above into the cylindrical case 1, it is possible to providethe hollow fiber membrane module M1 which is the object of the inventionand with which a pressure loss in water discharge at a high flow ratecan be reduced while damage to the hollow fiber membranes can beprevented.

On the other hand, although the hollow fiber membrane bundle 2 in whicheach of the hollow fiber membranes 2 a hangs downward from the hollowfiber membrane open end fixing member (upper fixing member) 13 a and hasthe sealed lower end is described above, a position of the lower end ofeach of the hollow fiber membranes 2 a in the cylindrical case 1 maybecome unstable due to movement of the raw water fed from an opening ata lower end of the cylindrical case 1 in this case. Therefore, it iscommon practice to provide a member for fixing the lower end of each ofthe hollow fiber membranes 2 a to the cylindrical case 1. Because thismember is employed in many conventional hollow fiber membrane modules,the member is briefly described here while referring to FIG. 1.

In other words, the cylindrical case 1 has the lower socket 4 b providedto its lower end. A feed port 10 for raw water is provided to a portionof an outer peripheral face of the lower socket 4 b. The lower end ofeach of the hollow fiber membranes 2 a is fixed by a hollow fibermembrane sealed end fixing member (lower fixing member) 13 b and thehollow portion at the lower end of each of the hollow fiber membranes 2a is sealed with resin forming the hollow fiber membrane sealed endfixing member (lower fixing member) 13 b in the hollow fiber membranesealed end fixing member (lower fixing member) 13 b to form a closedportion 2 aC.

An outer peripheral face of the hollow fiber membrane sealed end fixingmember (lower fixing member) 13 b is joined to an inner peripheral faceof a lower end of the lower socket 4 b. Between the feed port 10 of thelower socket 4 b and an outer peripheral face of the hollow fibermembrane bundle 2 facing the feed port 10, a lower flow straighteningcylinder 6 b having a large number of flow straightening holes 8 b isprovided and a lower end of the lower flow straightening cylinder 6 b isfixed to the hollow fiber membrane sealed end fixing member (lowerfixing member) 13 b.

Between the inner peripheral face of the lower socket 4 b and an outerperipheral face of the lower flow straightening cylinder 6 b, a lowerannular flow path 7 b is formed. An upper surface (inner surface) 13 bUSis formed on a surface of the hollow fiber membrane sealed end fixingmember (lower fixing member) 13 b and positioned inside the cylindricalcase 1 and a lower surface (outer surface) 13 bLS is formed on a surfaceof the hollow fiber membrane sealed end fixing member (lower fixingmember) 13 b and positioned outside the cylindrical case 1.

The raw water is fed from the feed port 10 for the raw water into thecylindrical case 1, flows through the lower annular flow path 7 b,passes through the flow straightening holes 8 b of the lower flowstraightening cylinder 6 b, and flows along outer surfaces of therespective hollow fiber membranes 2 a. On the other hand, if a lower cap5 b is provided to the lower end of the cylindrical case 1, it ispossible to feed the raw water into the cylindrical case 1 from a secondfeed port 12 provided to the lower cap 5 b. In this case, through holes14 through which the raw water can pass are provided to the hollow fibermembrane sealed end fixing member (lower fixing member) 13 b. The rawwater fed from the second feed port 12 passes through the through hales14 and flows along the outer surfaces of the respective hollow fibermembranes 2 a.

The raw water flowing along the outer surfaces of the respective hollowfiber membranes 2 a permeates through membrane walls of the respectivehollow fiber membranes 2 a to reach the hollow portions of the hollowfiber membranes to be filtrate. The filtrate passes through the openings2 aO of the respective hollow fiber membranes 2 a and is collected intoan upper cap 5 a provided to an upper end of the cylindrical case 1 andthen taken out of the hollow fiber membrane module M1 from a filtrateoutlet 11 provided to the upper cap 5 a.

In a dead-end filtration method, the discharge port 9 for the raw wateris closed and only the filtrate is taken out of the filtrate outlet 11.In a cross-flow method, on the other hand, the remaining raw water(concentrated water) which has not permeated through the hollow fibermembranes passes through the flow straightening holes 8 a of the upperflow straightening cylinder 6A which is provided to a peripheral face ofthe hollow fiber membrane bundle 2 at a position facing the dischargeport 9 for the raw water and has the large number of flow straighteningholes 8 a, passes through the upper annular flow path 7 a providedbetween the inner peripheral face of the upper socket 4 a and the outerperipheral face of the upper flow straightening cylinder 6A, and isdischarged out of the hollow fiber membrane module M1 from the dischargeport 9 for the raw water.

In this embodiment, the hollow fiber membrane open end fixing member(upper fixing member) 13 a and the hollow fiber membrane sealed endfixing member (lower fixing member) 13 b are formed by theconventionally-used potting method which has been described above andwhich uses the resin having the flowability.

In order to make the respective hollow fiber membranes 2 a open at theupper surface 13 aUS of the hollow fiber membrane open end fixing member(upper fixing member) 13 a, an adhesive is applied on an end face of thehollow fiber membrane bundle 2 to prevent a potting material fromentering the hollow portions of the respective hollow fiber membranes 2a before the potting and the potting material is fed by a centrifugalmethod. Then, after the potting material is cured, an opening-side endof the formed member may be cut off. To form the through holes 14 in thehollow fiber membrane sealed end fixing member (lower fixing member) 13b, metal pins or the like are arranged in positions for the potting andthe potting material is fed by the centrifugal method. Then, when thepotting material is cured, the pins may be pulled out.

In the hollow fiber membrane module M1 shown in FIG. 1, because aclearance is provided between an inner face of the upper flowstraightening cylinder 6A provided with the flow straightening holes 8 aand the outer face of the hollow fiber membrane bundle 2, the pressureloss in the water discharge at the high flow rate can be reduced whilethe damage to the hollow fiber membranes 2 a can be prevented. A shapeof the upper flow straightening cylinder 6A used in the hollow fibermembrane module M1 is described below in detail.

The upper flow straightening cylinder 6A used in the hollow fibermembrane module has the large inner diameter portion LDP and the smallinner diameter portion SDP having the smaller inner diameter SD than theinner diameter LD of the large inner diameter portion LDP and has a stepbetween the inner diameter SD of the small inner diameter portion SDPand the inner diameter LD of the large inner diameter portion LDP asshown as one example in FIG. 3. Because the small inner diameter portionSDP prevents increase in an outer diameter of the hollow fiber membranebundle 2, the clearance is formed between the flow straightening holes 8a of the upper flow straightening cylinder 6A and the hollow fibermembrane bundle 2. In this way, the hollow fiber membranes 2 a areprevented from closing the flow straightening holes 8 a and the pressureloss is maintained low. Moreover, because the hollow fiber membranes 2 aare less liable to come in contact with the flow straightening holes 8a, the damage to the hollow fiber membranes 2 a is prevented.

Furthermore, when an inner cross-sectional area of the small innerdiameter portion SDP is SA and an inner cross-sectional area of thelarge inner diameter portion LDP is LA, the ratio SA/LA preferablysatisfies a relationship of 0.6≦SA/LA≦50.9. If the ratio is smaller than0.6, an area of the hollow fiber membranes 2 a per unit area reduces. Onthe other hand, if the ratio exceeds 0.9, the clearance between the flowstraightening holes 8 a of the upper flow straightening cylinder 6A andthe hollow fiber membrane bundle 2 becomes small and the hollow fibermembranes 2 a may come in contact with the flow straightening holes 8 ato cause increase in the pressure loss and damage to the hollow fibermembranes 2 a in the water discharge in some cases.

Furthermore, preferably, the step is formed between the inner diameterof the small inner diameter portion SDP and the inner diameter of thelarge inner diameter portion LDP and the ratio SA/LA between thecross-sectional areas is obtained due to the step.

Here, the step means, on an inner surface of the upper flowstraightening cylinder 6A, a difference between the inner diameter ofthe small inner diameter portion SDP and the inner diameter of the largeinner diameter portion LDP when an inclination θ which a line segment SL(shown in a one-dot-chain line in FIG. 3) connecting an end (lower end)of the small inner diameter portion SDP on a side of the large innerdiameter portion LDP and an end (upper end) of the large inner diameterportion LDP on a side of the small inner diameter portion SDP in avertical section makes with respect to the axial direction of the upperflow straightening cylinder 6A is 45° or larger. A zone between a startpoint on the side of the small inner diameter portion SDP and an endpoint on the side of the large inner diameter portion LDP of the linesegment SL in the axial direction of the upper flow straighteningcylinder 6A is an inner diameter transition zone. Within the innerdiameter transition zone, a value of the inner diameter of the upperflow straightening cylinder 6A changes from the value of the innerdiameter of the small inner diameter portion SDP to the value of theinner diameter of the large inner diameter portion LDP continuously orstep by step.

The inclination θ is preferably 60° or larger. If the inclination θ is agentle inclination smaller than 45°, the hollow fiber membranes 2 a andthe upper flow straightening cylinder 6A come in contact with each otherto cause the damage to the hollow fiber membranes 2 a in some cases. Byobtaining the ratio SA/LA between the cross-sectional areas due to thestep having the inclination θ of 60° or larger, it is possible to moresatisfactorily prevent contact between the hollow fiber membranes 2 aand the flow straightening cylinder 6A.

It is preferred that a filling rate of the hollow fiber membranes 2 a inthe hollow fiber membrane bundle 2 in the small inner diameter portionSDP is in the range of 30% to 80%. If the filling rate is lower than30%, the filling rate of the hollow fiber membranes 2 a in the smallinner diameter portion SDP is low, the hollow fiber membranes 2 aoscillate excessively at the lower surface 13 aLS of the hollow fibermembrane open end fixing member (upper fixing member) 13 a, and thehollow fiber membranes 2 a at the lower surface 13 aLS may be cut insome cases during the water discharge at the high flow rate.

If the filling rate exceeds 80%, it may become difficult to insert thehollow fiber membrane bundle 2 into the cylindrical case 1 in amanufacturing process of the hollow fiber membrane module M1. Here, thefilling rate of the hollow fiber membranes 2 a in the hollow fibermembrane bundle 2 refers to a percentage of the total cross-sectionalarea of the hollow fiber membranes 2 a forming the hollow fiber membranebundle 2 in a cross-sectional area surrounded with the hollow fibermembranes 2 a positioned at an outermost periphery of the hollow fibermembrane bundle 2 in the small inner diameter portion SDP.

Preferably, the small inner diameter portion SDP is not provided withthe flow straightening holes 8 a and the large inner diameter portionLDP is provided with the flow straightening holes 8 a. In other words,all the flow straightening holes 8 a are preferably provided to thelarge inner diameter portion LDP. With this structure, the hollow fibermembranes 2 a positioned at the outer peripheral portion of the hollowfiber membrane bundle 2 are prevented from being pushed by the water orair and drawn into the flow straightening holes 8 a during the waterdischarge at the high flow rate.

In the hollow fiber membrane module in the invention, it is preferredthat the lower surface 13 aLS of the hollow fiber membrane open endfixing member (upper fixing member) 13 a fixing the hollow fibermembrane bundle 2 and the upper flow straightening cylinder 6A to theupper end of the cylindrical case 1 is positioned in an inner space ofthe small inner diameter portion SDP of the upper flow straighteningcylinder 6A. With this structure, the excessive oscillation of thehollow fiber membranes 2 a is prevented. As a result, the cut of thehollow fiber membranes 2 a at the lower surface 13 aLS of the hollowfiber membrane open end fixing member (upper fixing member) 13 a isprevented.

An inner surface of the small inner diameter portion SDP preferablyextends inward (downward) in the axial direction of the cylindrical case1 in the range of 3 mm to 40 mm of a length SDPL from the lower surface13 aLS. In other words, the length SDPL from the lower surface (innersurface) of the hollow fiber membrane open end fixing member (upperfixing member) to a lower end face of the small inner diameter portionin the axial direction of the upper flow straightening cylinder 6A ispreferably in the range of 3 mm to 40 mm.

If the hollow fiber membrane open end fixing member (upper fixingmember) 13 a is formed by the centrifugal potting, permeation of thepotting material (resin having the flowability) along the outer surfacesof the hollow fiber membranes 2 a can be kept below 3 mm. In the case ofthe stationary method, however, if the potting material (resin havingthe flowability) has a low viscosity, the permeation becomes 20 mm orgreater. By extending the small inner diameter portion SDP fartherinward (downward) than the permeation height of the lower surface 13aLS, the cut of the hollow fiber membranes 2 a at the uneven resinsurface is prevented. The small inner diameter portion SDP is preferablyextended inward (downward) 3 mm or greater from the permeation height.

On the other hand, if the lower surface 13 aLS is positioned at adistance longer than 40 mm from the lower end face of the small innerdiameter portion SDP, air becomes liable to accumulate in a clearancebetween the lower surface 13 aLS and the lower end face of the smallinner diameter portion SDP. As a result, an effective membrane area ofthe hollow fiber membranes 2 a reduces.

By setting an outer diameter of the flow straightening cylinder 6A atthe position of the small inner diameter portion SDP to a smaller valuethan an outer diameter of the flow straightening cylinder 6A at theposition of the large inner diameter portion LDP, it is possible towiden the upper annular flow path 7 a for discharging the water frominside the hollow fiber membrane module M1. As a result, the pressureloss in the flow of the raw water is reduced.

The material of the hollow fiber membranes 2 a forming the hollow fibermembrane bundle 2 is not particularly limited, and examples of thematerial include polysulfone, polyethersulfone, polyacrylonitrile,polyimide, polyetherimide, polyamide, polyether ketone, polyether etherketone, polyethylene, polypropylene, an ethylene-vinylalcohol copolymer,cellulose, cellulose acetate, polyvinylidene fluoride, anethylene-tetrafluoroethylene copolymer, polytetrafluoroethylene, and acomposite material of these materials. Of these, polyvinylidene fluoridehas excellent chemical resistance and, hence, if periodical washing ofthe hollow fiber membrane module M1 with a chemical is conducted torecover the filtration function of the hollow fiber membranes 2 a, useof polyvinylidene fluoride leads to prolongation of the life of thehollow fiber membrane module M1. Therefore, polyvinylidene fluoride isused preferably as the material of the hollow fiber membrane.

An outer diameter of each of the hollow fiber membranes 2 a ispreferably in the range of 0.3 to 3 mm. If the outer diameter of thehollow fiber membrane 2 a is excessively small, the hollow fibermembrane 2 a may be bent and damaged during manufacture of the hollowfiber membrane module M1, in which the hollow fiber membrane 2 a istreated, and during use of the hollow fiber membrane module M1, in whichthe filtration and the washing are conducted. If the outer diameter ofthe hollow fiber membrane 2 a is excessively large, on the other hand,the number of hollow fiber membranes 2 a which can be inserted into thecylindrical case 1 of the same size reduces, resulting in reduction in afiltration area in some cases.

A membrane thickness of each of the hollow fiber membranes 2 a ispreferably in the range of 0.1 to 1 mm. If the membrane thickness of thehollow fiber membrane 2 a is excessively small, the hollow fibermembrane 2 a may be bent due to pressure during use of the hollow fibermembrane module M1. If the membrane thickness of the hollow fibermembrane 2 a is excessively large, on the other hand, it may lead toincrease in the pressure loss during use of the hollow fiber membranemodule M1 and increase in material cost.

As values of physical properties of the hollow fiber membrane 2 a, it ispreferred to take a longitudinal elastic modulus and second moment ofarea into consideration. For example, if the hollow fiber membrane 2 ahaving the outer diameter of 1.5 mm, the inner diameter of 1 mm, thesecond moment of area of 0.2 mm⁴, the longitudinal elastic modulus of200 MPa, the membrane length of 2000 mm, and the transmembrane pressuredifference of 100 kPa during water discharge is used, a clearance ofabout 10 mm may be provided between the hollow fiber membrane bundle 2and the flow straightening holes 8 a.

If the hollow fiber membrane 2 a is a weak hollow fiber membrane havingsmall longitudinal elastic modulus and second moment of area, it ispossible to reduce the pressure loss in the water discharge at the highflow rate while preventing damage to hollow fiber membrane, when thefollowing relationship is satisfied.

In other words, when the inner diameter of the small inner diameterportion SDP is SD, the inner diameter of the large inner diameterportion LDP is LD, the length of the hollow fiber membrane 2 a betweenthe lower surface (inner surface) 13 aLS of the hollow fiber membraneopen end fixing member (upper fixing member) 13 a and the upper surface(inner surface) 13 bUS of the hollow fiber membrane sealed end fixingmember (lower fixing member) 13 b is FL, and a distance in the axialdirection of the cylindrical case 1 between the lower surface (innersurface) 13 aLS of the hollow fiber membrane open end fixing member(upper fixing member) 13 a and the upper surface (inner surface) 13 bUSof the hollow fiber membrane sealed end fixing member (lower fixingmember) 13 b is CL, it is preferred that the relationship,LD−SD>(FL−CL)/2 is satisfied.

As materials forming the case main body 3, the upper socket 4 a, thelower socket 4 b, the upper cap 5 a, the lower cap 5 b, the upper flowstraightening cylinder 6A, and the lower flow straightening cylinder 6b, the following materials can be used, for example.

Examples of the material for forming these members include polyolefinssuch as polyethylene, polypropylene, and polybutene, fluorine resinssuch as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),ethylene-tetrafluoroethylene copolymer (ETFE),polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylenecopolymer (ECTFE), and vinylidene fluoride (PVDF), chloride resins suchas polyvinyl chloride and polyvinylidene chloride, and a polysulfoneresin, a polyethersulfone resin, a polyallylsulfone resin, apolyphenylether resin, an acrylonitrile-butadiene-styrene copolymerresin (ABS), an acrylonitrile-styrene copolymer resin, apolyphenylenesulfide resin, a polyamide resin, a polycarbonate resin, apolyether ketone resin, and a polyether ether ketone resin. These resinscan be used alone or as a mixture thereof.

Other than the resins, aluminum, stainless steel, and the like arepreferred and composite materials such as a composite material of aresin and a metal, glass fiber-reinforced resins, and carbonfiber-reinforced resins are used.

The case main body 3, the upper socket 4 a, the lower socket 4 b, theupper cap 5 a, the lower cap 5 b, the upper flow straightening cylinder6A, and the lower flow straightening cylinder 6 b may be made of thesame material or different materials.

Size of the hollow fiber membrane module M1 is not particularly limited.In other words, the outer diameter and the axial length, for example,are not particularly limited. However, because a module with greaterbore and length has a larger filtration area per unit area in general,such a module is used preferably. With the module with greater bore andlength, a water treatment amount per unit time increases and thereforethe effects of the invention are exerted sufficiently.

As the potting material (resin) for forming the hollow fiber membraneopen end fixing member (upper fixing member) 13 a and the hollow fibermembrane sealed end fixing member (lower fixing member) 13 b, athermosetting resin which is liquid initially is used preferably.

Since the potting material (resin) is required to have the strengthrequired for tube plates that separate the inside of the hollow fibermembrane bundle 2 from the outside thereof, it is preferred that thepotting material (resin) has a D hardness of 50 to 85. D hardness is avalue measured after 10-second indentation with a type D durometer inaccordance with JIS K 6253. Specific examples of the resin include epoxyand polyurethane. Each fixing member need not be a single layerconstituted of one resin, and may be configured of a plurality oflayers.

Although the hollow fiber membrane bundle 2 in which the respectivehollow fiber membranes 2 a are positioned straight in the verticaldirection and which have the opposite ends respectively fixed by thefixing members 13 a and 13 b to the upper portion and the lower portionof the cylindrical case 1 has been described, a hollow fiber membranemodule in which a hollow fiber membrane bundle is bent into a U shapeand has open ends fixed to a fixing member 13 a may be employed.Alternatively, what is called a cartridge-type hollow fiber membranemodule in which a hollow fiber membrane cartridge configured of a hollowfiber membrane bundle is mounted into a housing may be employed.

Example 1

About 9,000 hollow fiber membranes, each of which was made ofpolyvinylidene fluoride (inner diameter, 1.0 mm; outer diameter, 1.5 mm)and had a length of about 2,000 mm, were bundled, and the resultant wasinserted into a casing made of polyvinyl chloride and having an innerdiameter of about 200 mm. A urethane resin which had a D hardness of 75and which had been formed using a modified polyisocyanate and a modifiedpolyol as starting materials was cast to fix both ends of the bundle bya centrifugal method so that the resin had a thickness of about 100 mmon each of the open side and the sealed side. Thus, an external-pressuretype hollow fiber membrane module M1 shown in FIG. 1 was produced.

A length FL of each of the hollow fiber membranes 2 a between a lowersurface 13 aLS of a hollow fiber membrane open end fixing member (upperfixing member) 13 a and an upper surface 13 bUS of a hollow fibermembrane sealed end fixing member (lower fixing member) 13 b was 1,810mm and a distance CL, in an axial direction of the cylindrical case 1,between the lower surface 13 aLS and the upper surface 13 bUS was 1,800mm. An inner diameter SD of a small inner diameter portion SDP of anupper flow straightening cylinder 6A was 175 mm and an inner diameter LDof a large inner diameter portion LDP was 195 mm. These values satisfieda relationship, LD−SD>(FL−CL)/2.

The number of flow straightening holes 8 a having a diameter of 5 mm andprovided to the large inner diameter portion LDP was 350. A pitch of theflow straightening holes 8 a in a direction parallel to an inserteddirection (vertical direction of the upper flow straightening cylinder6A) of the hollow fiber membranes was 10 mm and a pitch in a verticaldirection (circumferential direction of the upper flow straighteningcylinder 6A) was 15 mm. The small inner diameter portion SDP of theupper flow straightening cylinder 6A was extended 5 mm inward (downward)in the axial direction of the cylindrical case 1 from the lower surface13 aLS.

By using the hollow fiber membrane module M1, a filtrate manufacturingoperation test was conducted. As raw water, water from Lake Biwa wassubjected to constant flow rate filtration at a membrane filtration fluxof 3.0 m³/(m²·d). After 30 minutes since the start of the filtration, anair-scrubbing and back-pressure simultaneous washing step was conducted,in which washing water for back-pressure washing was fed from a filtrateoutlet 11 at a membrane filtration flux of 4.5 m³/(m²·d), air was fedfrom a feed port 12 at 100 L/min, and back-pressure washing water andair was caused to flow out from a discharge port 9 was conducted for oneminute. The filtration step and the air-scrubbing and back-pressuresimultaneous washing process were conducted repeatedly.

As a result, a membrane filtration pressure difference in theair-scrubbing and back-pressure simultaneous washing step of the hollowfiber membrane module M1, that was 70 kPa immediately after the start ofthe operation, was still 90 kPa after one year and it was possible toconduct stable operation.

In order to check if the hollow fiber membranes 2 a were cut in themodule M1, an air leak inspection was conducted. In the air leakinspection, the module was completely filled with water both on araw-water side and on a filtrate side, 0.1 MPa compressed air wasthereafter supplied from outside the hollow fiber membranes, and thehollow fiber membranes were visually examined as to whether bubblingfrom the openings 2 aO thereof occurred or not. As a result of the airleak inspection, bubbling from the openings 2 aO of the hollow fibermembranes was not observed and it was confirmed that the hollow fibermembranes 2 a in the module M1 were not damaged.

When the module M1 after the operation was disassembled, no mark showingcontact of the hollow fiber membranes 2 a with the flow straighteningholes 8 a was found on the hollow fiber membranes 2 a near the flowstraightening holes 8 a.

Example 2

An upper flow straightening cylinder 6A having a small inner diameterportion SDP with an inner diameter SD of 189 mm and a large innerdiameter portion LDP with an inner diameter LD of 195 mm was used. Thesevalues satisfied a relationship, LD−SD>(FL−CL)/2. Except these changes,the same hollow fiber membrane module M1 as in Example 1 was producedand the same operation test as in Example 1 was conducted.

As a result, a membrane filtration pressure difference in air-scrubbingand back-pressure simultaneous washing of the hollow fiber membranemodule M1, that was 120 kPa immediately after the start of theoperation, was 150 kPa after six months and it was possible to conductstable operation.

As a result of the air leak inspection, bubbling from openings 2 aO ofhollow fiber membranes 2 a was not observed and it was confirmed thatthe hollow fiber membranes 2 a in the module M1 were not damaged.

When the module M1 after the operation was disassembled, no mark showingcontact of the hollow fiber membranes 2 a with flow straightening holes8 a was found on the hollow fiber membranes 2 a near the flowstraightening holes 8 a.

Comparative Example 1

The same hollow fiber membrane module as in Example 1 was produced,except that a hollow fiber membrane open end fixing member (upper fixingmember) 13 a had a thickness of about 110 mm and that a lower surface 13aLS was disposed at a large inner diameter portion LDP, and the sameoperation test as in Example 1 was conducted.

As a result, a membrane filtration pressure difference in air-scrubbingand back-pressure simultaneous washing of the hollow fiber membranemodule, that was 70 kPa immediately after the start of the operation,was still 95 kPa after one year and it was possible to conduct stableoperation.

As a result of the air leak inspection, however, leaks from eightpositions were found from openings 2 aO of hollow fiber membranes 2 a.

When the module after the operation was disassembled, no mark showingcontact of the hollow fiber membranes 2 a with flow straightening holes8 a was found on the hollow fiber membranes 2 a near the flowstraightening holes 8 a. However, it was confirmed that the hollow fibermembranes 2 a was cut at a lower surface 13 aLS.

Comparative Example 2

The same hollow fiber membrane module as in Example 1 was produced,except that a straight flow straightening cylinder having an innerdiameter of 195 mm was used, and the same operation test as in Example 1was conducted.

As a result, a membrane filtration pressure difference in air-scrubbingand back-pressure simultaneous washing of the hollow fiber membranemodule, that was 200 kPa immediately after the start of the operation,was 300 kPa after three months and it was impossible to conduct stableoperation.

As a result of the air leak inspection, leaks from 10 positions werefound from openings 2 aO of hollow fiber membranes 2 a. All the leakpositions were at an outer periphery of the hollow fiber membranebundle.

When the module after the operation was disassembled, marks showing thatthe hollow fiber membranes 2 a near flow straightening holes 8 a hadcome in contact with the flow straightening holes 8 a and gotten shavedwere found and the leaks from the positions were confirmed.

The hollow fiber membrane module according to the invention includes ahollow fiber membrane module with the reduced pressure loss in the waterdischarge at the high flow rate and less damage to the membranes and issuitably used for a water purification treatment, production ofindustrial water, a wastewater treatment, a pretreatment of raw water ina water treatment using reverse osmosis membranes.

DESCRIPTION OF REFERENCE SIGNS

-   -   1: Cylindrical case    -   2: Hollow fiber membrane bundle    -   2 a: Hollow fiber membrane    -   2 aO: Opening    -   2 aC: Closed portion    -   3: Case main body    -   4 a: Upper socket    -   4 b: Lower socket    -   5 a: Upper cap    -   5 b: Lower cap    -   6A: Upper flow straightening cylinder    -   6 a: Upper flow straightening cylinder    -   6 b: Lower flow straightening cylinder    -   7 a: Upper annular flow path    -   7 b: Lower annular flow path    -   8 a, 8 b: Flow straightening holes    -   9: Discharge port (nozzle)    -   10: Feed port    -   11: Filtrate outlet    -   12: Second feed port    -   13 a: Hollow fiber membrane open end fixing member (upper fixing        member)    -   13 aLS: Lower surface (inner surface)    -   13 aUS: Upper surface (Outer surface)    -   13 b: Hollow fiber membrane sealed end fixing member (Lower        fixing member)    -   13 bLS: Lower surface (Outer surface)    -   13 bUS: Upper surface (Inner surface)    -   14: Through hole    -   CL: Distance    -   FL: Length of hollow fiber membrane    -   LA: Cross-sectional area of large inner diameter portion    -   LD: Inner diameter of large inner diameter portion    -   LDP: Large inner diameter portion    -   M1: Hollow fiber membrane module    -   M4: Hollow fiber membrane module    -   SA: Cross-sectional area of small inner diameter portion    -   SD: Inner diameter of small inner diameter portion    -   SDP: Small inner diameter portion    -   SDPL: Length from lower surface (inner surface) of hollow fiber        membrane open end fixing member (upper fixing member) to lower        end face of small inner diameter portion    -   SL: Line segment showing inner diameter transition zone    -   θ: Inclination

The invention claimed is:
 1. A hollow fiber membrane module including: acylindrical case having one nozzle at the minimum in a side facethereof; a hollow fiber membrane bundle provided in the cylindricalcase; and a flow straightening cylinder provided between the one nozzleat the minimum and an outer periphery of the hollow fiber membranebundle and having a plurality of flow straightening holes, in which thehollow fiber membrane bundle includes a plurality of hollow fibermembranes, that are open at one end and sealed at the other end, or atleast one U-shaped hollow fiber membrane, that is open at opposite ends,and includes a hollow fiber membrane open end fixing member for fixingthe open end of each of the hollow fiber membranes, a side peripheralface of the hollow fiber membrane open end fixing member is joined to aninner peripheral face of the cylindrical case, each of the hollow fibermembranes passes through the hollow fiber membrane open end fixingmember, each of the open ends is positioned at an outer surface of thehollow fiber membrane open end fixing member, the outer surfacepositioned outside the cylindrical case, and the flow straighteningcylinder is fixed to the hollow fiber membrane open end fixing member,wherein the flow straightening cylinder includes a large inner diameterportion and a small inner diameter portion having a smaller innerdiameter than an inner diameter of the large inner diameter portion atleast at a part in an axial direction of the flow straighteningcylinder, an inner surface of the hollow fiber membrane open end fixingmember positioned on an opposite side from the outer surface and insidethe cylindrical case is positioned at the small inner diameter portionof the flow straightening cylinder, the flow straightening cylinder hasa step between the small inner diameter portion and the large innerdiameter portion, and clearance is formed between the flow straighteningholes of the flow straightening cylinder and the hollow fiber membranebundle.
 2. The hollow fiber membrane module according to claim 1,wherein the hollow fiber membrane bundle includes a plurality of hollowfiber membranes that are open at one ends and sealed at the other ends,the sealed end of each of the hollow fiber membranes is fixed to ahollow fiber membrane sealed end fixing member, a side peripheral faceof the hollow fiber membrane sealed end fixing member is joined to aninner peripheral face of the cylindrical case, and the hollow fibermembrane sealed end fixing member has an inner surface positioned insidethe cylindrical case and an outer surface positioned outside thecylindrical case.
 3. The hollow fiber membrane module according to claim2, wherein, when the inner diameter of the large inner diameter portionis LD, the inner diameter of the small inner diameter portion is SD, ahollow fiber membrane length between the inner surface of the hollowfiber membrane open end fixing member and the inner surface of thehollow fiber membrane sealed end fixing member is FL, and a distance, inthe axial direction, of the cylindrical case between the inner surfaceof the hollow fiber membrane open end fixing member and the innersurface of the hollow fiber membrane sealed end fixing member is CL, theflow straightening cylinder satisfies a relationship, (LD−SD)>(FL−CL)/2.4. The hollow fiber membrane module according to claim 1, wherein alength, in an axial direction of the cylindrical case, of the smallinner diameter portion from the inner surface of the hollow fibermembrane open end fixing member toward the large inner diameter portionis in the range of 3 mm to 40 mm.
 5. The hollow fiber membrane moduleaccording to claim 1, wherein, when a cross-sectional area of the smallinner diameter portion of the flow straightening cylinder is SA and across-sectional area of the large inner diameter portion is LA, a ratiobetween SA and LA satisfies a relationship, 0.6≦SA/LA≦0.9.
 6. The hollowfiber membrane module according to claim 5, wherein the flowstraightening cylinder has an inner diameter length transition zone inwhich an inner diameter of the flow straightening cylinder changes froman inner diameter satisfying the cross sectional area SA of the smallinner diameter portion to an inner diameter satisfying thecross-sectional area LA of the large inner diameter portion between thesmall inner diameter portion and the large inner diameter portion. 7.The hollow fiber membrane module according to claim 1, wherein all ofthe plurality of flow straightening holes of the flow straighteningcylinder are provided at the large inner diameter portion.
 8. The hollowfiber membrane module according to claim 1, wherein a filling rate ofthe hollow fiber membranes in the hollow fiber membrane bundlepositioned at the small inner diameter portion is in the range of 30% to80%.
 9. The hollow fiber membrane module according to claim 1, whereinan outer diameter of the flow straightening cylinder at a portion wherethe small inner diameter portion is formed is smaller than an outerdiameter of the flow straightening cylinder at a portion where the largeinner diameter portion is formed.